Analysis and design of a branch-line balun with high-isolation wideband characteristics

This article presents a design of wideband balun with high isolation using a branch-line structure. Theoretical analysis of the proposed balun shows that the reflection coefficient characteristics with two transmission poles can be obtained by controlling the characteristic impedances of transmission lines (Z_t, Z₁, and Z₂). The high isolation can be obtained by adding a shunt coupled-line short stub and resistor (R) between the output ports. The proposed balun was designed at the center frequency (f₀) of 2.6 GHz for a current-mode class-S amplifier application. The measured results were in good agreement with the simulations, showing that power division ratios were 3.09 dB and 3.14 dB, whereas the return loss was 21.39 dB at the f₀ and higher than 20 dB over a bandwidth of 0.98 GHz (1.98-2.96 GHz). The isolation between output ports was higher than 18 dB for the bandwidth of 0.75 GHz (2.21-2.96 GHz). The measured phase difference between the output ports was 180 ± 9 over a frequency range of 2.2-2.98 GHz.